As disclosed in U.S. Pat. Nos. 3,888,016; 3,772,503 and 3,927,474, incorporated herein by reference, positional indicators and more particularly digital compasses had been provided in which the angular orientation of the compass card is determined by a primary optical pickoff which involves an encoder disc driven by the magnetic sensing element of the compass. The encoded disc is provided with digital encoder markings or a digital code that in general results in establishment of the angular position of the encoded disc and thus the angular position of the magnetic element of the compass. The accuracy to which position can be ascertained with a digitally encoded compass is on the order of 2.degree. (for example, .+-.1.degree.).
U.S. Pat. No. 3,772,503 discloses a typical usage of such a compass card technique. According to this patent, an inner pattern on the compass card would typically be a "Gray" code with the rotational position of the disc being detected by detector array positioned on one side of the disc so that it detects the light transmissive and light opaque portions of the inner pattern (i.e. the Gray code). To this end a light source is positioned to one side of the coded disc and the detector array is positioned on the other side along a radius of the disc. The transmissive pattern or Gray scale detected by the detector array is then decoded in a conventional manner and provides an output corresponding to the angular position of the disc with an accuracy of for example, 2.degree.. It will be appreciated of course, that resolution of the Gray code, and of most digital coding systems is determined by that portion of the pattern which represents the least significant bit. In the example discussed in U.S. Pat. No. 3,772,503, the light and dark areas which are at the peripheral of the inner pattern determine the resolution of the primary optical pickoff system.
According to another U.S. Pat. No. 4,414,754 by the same inventor as the present application, there is disclosed another improved technique which uses a second analog optical pickoff array which works in combinaton with the digital pickoff array to achieve substantial increased accuracy.
However, although these improved techniques have resulted in a great reduction in the size of optical compasses, the resolution is still effectively determined by the number of digital pattern rings located on the compass card. And to date, optical digital compass cards are limited to a realistic size of somewhat less than about one inch. Even the technique of U.S. Pat. No. 4,414,754 discussed above which allows significantly greater accuracy, requires an increased diameter of the compass card to allow for the presentation of the analog portion of the compass card.
Therefore, it is an object of this invention to provide an optical compass having extreme accuracy while at the same time allowing minimal size of the compass.
Other prior art patents which may be considered somewhat related to the present invention include such U.S. patents as U.S. Pat. Nos. 2,604,528 issued to A. A. Obermaier on July 22, 1952; 3,254,226 issued to A. Bobula et al on May 31, 1966; and 3,327,125 issued to L. W. Herchenroeder on June 20, 1967. Each of these patents discloses detection of radiant energy as it passes through a rotating disc. In addition, each of the patents includes the idea of the radient energy being transmitted through the disc in transparent or translucent areas of the disc which are arranged in a spiral manner on the surface of the disc.
U.S. Pat. Nos. 4,031,630 and 4,091,543 each assigned to the same assignee as the present application disclose the use of a ROM (read only memory) to store compass deviation corrections which are then combined with the actual compass reading to provide a corrected reading. However, it will be appreciated by those skilled in the art that the techniques disclosed in these two patents cannot improve the overall resolution of the digital compass but only provide deviation corrections to those readings provided by the digital compass. To achieve greater accuracy and resolution of optical compasses, or alternately to reduce the size while maintaining the present resolution and/or accuracy of optical compasses, already requires meticulous and careful placing of the individual elements in manufacturing the code cards and positioning the pickup arrays and the bearing position of the rotating compass card. That is, there is very little room for error in placing the pickup arrays and the compass card bearings if the desired accuracy is to be achieved. Consequently, to increase resolution or decrease the actual size of the compass card while maintaining the present accuracy could make the task even more difficult. Therefore, it is another object of the present invention to provide a high resolution, miniaturized and highly accurate compass card which does not require the careful and meticulous manufacturing techniques of the prior art optical compasses.
It is still another object of the present invention to provide an inexpensive and simplified method of manufacturing high resolution optical compasses.
Other objects and advantages will be obvious, and will in part appear hereinafter, and will be accomplished by the present invention which provides a system for determining the positional relationship between a first and second member suitable for a limited motion there between. An example of such first and second members include a magnetic compass card and a housing thereof. The system of this invention includes such a first member having a multiplicity of means such as, a photoelectric pickup array for detecting light or radiant energy. Each of the multiplicity of detectors in the array provides an output signal indicative of whether or not radiant energy has impinged upon that detector. It is also important, that each of the optical pickup elements or detectors in the array be maintained in a fixed position relationship with every other element of the array. Also included is a source of light or radiant energy positioned so as to direct radiant energy toward the array so that the energy will be received by each element of the array so long as an obstruction is not between the two. A second member, such as for example, a rotating disc with a mask having transmissive areas is positioned between the multiplicity of optical pickup elements or detectors and the radiant energy source. This mask on the rotating disc blocks the radiant energy from impinging on the multiplicity of light detectors except for that radiant energy which is allowed to pass through the transmission areas or slits on the mask. Thus, only a portion of the radiant energy from the source will be transmitted through the rotatable disc. Therefore, the specific ones of the multiplicity of optical pickup elements or light detectors receiving the radiant energy will depend upon the relative position of the rotating disc or second member with respect to the position of the first member which supports the array of photodetectors. There is also included a means (such as a Read Only Memory) for storing and providing, upon command, information representative of a multiplicity of specific relative positions between the first and second members and a specific combination of activated optical pickup elements for each such position. These specific relative positions are determined as a function of the specific ones of the photoelectric pickup elements which receive the impinging radiant energy from the source for each position. Thus, there is a different combination of activated radiant energy monitors or pickups for each specific position. During operation, the combination of specific optical pickups which are activated by the radiant energy source at a specific location of the second member are then compared with the stored information until a match is found. Once the match is achieved, the specific relative position of the first member to the second member is known. In a preferred embodiment, the mask or second member will rotate in response to a magnetic compass, and the first member will function as the compass housing secured in a fixed manner or position to a moving vehicle such as a ship. Thus, the magnetic heading of the vehicle or ship can be determined.